1. Technical Field
The invention relates to the field of strain sensors and, in particular, to temperature compensated strain sensors making use of optical fiber technology suitable for embedding in molded articles such as composite structures.
2. Background Art
Composite parts are generally composed of filamentary material in a resin matrix. The filamentary material is either woven or unidirectional and the orientation thereof is dictated by design considerations. The part is typically made by laying up individual sheets of pre-impregnated filamentary material in a mold. The lay up is vacuum bagged and a vacuum is drawn from between the bag and mold and the whole assembly is placed in an autoclave or heated press. Thereafter, the temperature and pressure are raised to mold the part. If the resin is a thermoplastic type, it is typically heated to a point where the resin melts and flows to form the homogenous matrix and if it is a thermoset type resin it is heated to a temperature wherein it melts and cures.
The embedding of optical fibers in composite structures to measure strain is not new. For example, U.S. Pat. No. 3,910,105 "Method for Detection Of Flaws In Composite Fiberglass Structures" by D. J. Hoffstedt discloses a method of embedding optical fibers in a composite structure such as a rotor blade for a helicopter. Numerous optical fibers are placed in the structure during fabrication such that the ends exit thereof. If during normal flight operations the rotor blade is overstressed or becomes fatigue damaged, one or more of the optical fibers will be damaged or even severed. The damage is easily detected by ducting a light beam through the fibers, for the light will be attenuated if the fiber is cracked or broken. However, this invention only really indicates a strain level that has caused damage and is incapable of providing a quantitative value of the strain level at any particular time.
Applicant's invention, U.S. Pat. No. 4,836,030 "Method Of Testing Composite Materials For Structural Damage" provides an improved method of embedding the optical fibers. In this method the optical fibers are first embedded in a layer of resin prior to the lay up of the structure. Thus, when the actual lay up of the structure was undertaken, the optical fibers being already positioned in the resin sheet, are automatically positioned in the lay up upon installation of the resin sheet. However, there was still no capability to measure strain levels below a level that caused damage.
Several fiber optic sensing techniques have been devised which can detect optical signal changes resulting from strain imposed anywhere along the length of an imbedded optical fiber. For example, a high resolution optical time domain sensing technique has been disclosed by B. Zimmermann et al. in an article entitled, "Fiber Optic Sensors Using High Resolution Optical Time Domain Instrumentation Systems" OFC Conference (San Francisco) January 1990. In the Zimmermann article a high resolution optical time domain reflectometer (OTDR) is used in a transmissive mode and utilizes a re-entrant loop coupler to further increase resolution of very small changes in fiber length to enable embedded fiber strain measurement. A light pulse is injected by a coupler into a loop of optical fibers to recirculate repeatedly through the loop. Each time the pulse passes the coupler, a small portion of the pulse is diverted to a detector which has measured elapsed time since injection of the original light pulse. The detector records the resultant string of small portions of the original pulse, all measured in the time domain, which (according to the speed of light in glass fiber) can be converted to a length measurement of the fiber loop multiplied by the number of detectable pulse portions. Using this method the increase or decrease in length (strain) of the embedded fiber can be effectively measured which can be used to calculate the stress and ultimately the load. In practical applications however, different strains can be imposed at two or more sites distributed anywhere along the fiber length. Such distributed sensing techniques can provide only one value, which represents the average of total strain effect all along the entire length of the fiber. In most cases, such a value is not very useful, since localized strain at specific locations is needed to determine critical strain loads. Furthermore, a change in temperature will also cause a change in the length of the optical fiber which will, in turn, cause the OTDR to give an inaccurate strain measurement. Up until the present invention there was no efficient way for measuring the strain by use of distributed optical fiber type sensors at a number of locations within a composite structure while simultaneously compensating for temperature.
In the following articles entitled, "Embedded Optical Fiber Strain Sensor for Composite Structure Applications", Fiber Optic and Laser Sensors IV, SPIE 718, 1987, by W. J. Rowe et al., and "A Rugged Optical Fiber Interferometer for Strain Measurements Inside a Composite Materials Laminate" by K. Murphy et al., and also in the previously mentioned article by B. Zimmermann, all three articles disclose distributed methods of sensing strain.
Thus, it is a primary object of the subject invention to provide a temperature compensated strain sensor.
It is another primary object of the subject invention to provide a temperature compensated strain sensor using distributed optical fiber technology.
It is another primary object of the subject invention to provide a temperature compensated strain sensor that can be embedded into a composite structure during the forming thereof.
It is a further object of the invention to provide a temperature compensated strain sensor that can be embedded within a composite structure that does not significantly effect the strength of the structure.